Vacuum flask



FIG. 1 is front elevation view of a vacuum flask showing my new design;

FIG. 2 is a rear elevation view thereof;

FIG. 3 is a left side elevation view thereof;

FIG. 4 is a right side elevation view thereof;

FIG. 5 is a top plan view thereof; and,

FIG. 6 is bottom plan view thereof. 

The ornamental design for a vacuum flask, as shown and described. 